An organ reed - that is, a reed with a vibrator larger than its aperture - produces a more powerful sound than any instrument of the flue-pipe variety. The wind pressure in each case being equal, a low note can be heard at a greater distance from its source than a high note, but a low note requires a larger tube. A note within the limits of a man's voice, say low F, would be suitable. This note could be produced with a tube about 3ft. long. A great pressure of wind is not required. The most powerful organ pipes speak under a pressure of about the weight of 12 in. of water, that is, about 631b. to the square foot, but everything depends on the weight and flexibility of the vibrator. The conical tube used for a speaking trumpet is a suitable shape for a mouthpiece. Two instruments could be adopted, which may be used either together or alternately. A short sound followed by silence is better than a continuous note. If two notes are used together, they may be nearly alike as is the duplex whistle used by the police, or they may be tuned in the interval of a third major or minor.

The combination of two sounds nearly alike gives rise to "beats," which are very elfective as "noises." With two sounds representing the dot and dash of the Morse alphabet any signal can be transmitted.